Measurement of heat transport

The Nusselt number Nu is defined asEmbedded Image(5)where J0 is the net heat flux passing through the convection cell. In the experiment, the temperature of the conducting plates is determined by the average readings of the two thermistors embedded in each plate. The value of J0 is given byEmbedded Image(6)where S = WD is the cross-sectional area of the cell at the middle height, Pin is the heating power applied to the bottom plate, and Pb and Ps represent the losses of the heating power through the bottom plate and the sidewalls, respectively. The bottom copper plate and Plexiglas sidewalls were encapsulated by a thermal insulation layer made of polystyrene foam. Two thermistors were placed across the insulation layer in each part to measure the conductive heat losses Pb and Ps.

Figure S2 shows the measured Nusselt number Nu as a function of Ra for water (Pr = 4.4) in the thin disc cell. It is seen that the measured Nu(Ra) is well described by the power law given in Eq. 4 (solid line).